As is well known, utility power is produced by a number of different power generators and delivered to end customers through a system of power transmission and distribution lines. Power transmission lines are generally high voltage lines used to transmit power from power generators to locations (e.g. power substations) where the power may be distributed through lower voltage power distribution lines to end customers. Transmission line power throughput is typically constrained by heating of the line which results in thermal expansion of the conductor, lengthening it, and manifesting itself as increased sag in the line. Regulations require transmission system operators to control the amount of power throughput (i.e., ampacity) in the lines such that the lines do not heat to the point that thermal expansion results in line sag where clearance with underlying features is reduced to the point that an unsafe condition occurs (e.g., line contacting underlying vegetation or structures). For safe operation, minimum clearances for transmission lines are specified, and the amount of power that can be transmitted through a line is regulated conservatively by the specification of a line rating. Line ratings are traditionally established for each line with conservative assumptions about ambient temperature (e.g., assumed to be relatively hot), wind speed (e.g., little wind for convective cooling of the line), and cloud cover (e.g., no cloud cover to mitigate direct sunlight from heating the lines). Ambient conditions can have a large influence on how much power can be transmitted through a power line while complying with clearance requirements.
Power utilities have historically used static line ratings to manage the capacity of their transmission lines to transmit power. Static line ratings are traditionally designed to be conservative, and generally reserve 30 percent or more of a lines capacity as buffer to avoid problems associated with line sag and clearance issues that can occur when certain environmental and load conditions are met.
Institute of Electrical and Electronics Engineers (IEEE) Standard 738 provides a mathematical equation used to estimate the relationship of conductor temperature (i.e., transmission line temperature) and current (i.e., electric load) with consideration of different weather conditions (variables to include ambient temperature, wind direction and magnitude, solar radiation influx). The primary use of the IEEE Standard 738 mathematical model is to establish a static line rating for the purpose of regulating the safe operation of power transmission lines.
As mentioned above, transmission line throughput is typically constrained by heating of the line. There are two primary criteria governing the safe functioning and operation of a power line: 1) the temperature of the conductor material must be maintained below a threshold such that the mechanical strength of the conductor itself is not compromised (i.e., mechanical properties of line do not deteriorate); 2) the load on the line must be maintained below a threshold such that as the temperature of the line increases with the load, and the line expands thermally, and thus lengthens, the sag of the line does not violate the minimum safe operating clearance with the ground. The first criterion is established by the known mechanical properties of the conductor material as determined through controlled laboratory studies, and is characteristic of a given type of conductor. The second criterion is satisfied by operating the line such that the static line rating as calculated using the IEEE Standard 738 equation is not exceeded.
The temperature of a power line is dynamic, and is affected by a number of variables to include the load on the line (i.e., electrical current transmitted through the line), convective cooling from the wind, conductive cooling/heating from ambient air temperatures, radiation cooling, and heating from incident solar radiation. The IEEE Standard 738 analytical model is a heat balance model that combines theory with empirical data to approximate the relationship of conductor temperature relative to load for a variety of environmental conditions. In practice, it is used to calculate the maximum permissible power load for a subject line (i.e., the line ampacity) using a maximum permitted conductor temperature (characteristic of the conductor material to avoid loss of strength, often set at 100° C. for aluminum clad steel reinforced conductor, for example) and a set of conservative (i.e, not conducive to line cooling) weather conditions (e.g., little wind, high ambient temperature, high incident solar radiation). The maximum power load calculated using the IEEE Standard 738 mathematical equation for the conditions described above is termed the static line rating, and when established for all the lines within a transmission system, is used by transmission operators to govern the safe operation of the grid. In other words, if a transmission system is operated such that none of the lines exceed their associated static line ratings, the operator is assured that the temperature of the conductors will not exceed a threshold that compromises their mechanical strength properties, and the sag of the line will not increase to the point that minimum ground clearances are violated.
The Power Delivery Industry has known for many years that using static line ratings to govern operation of the grid, while safe, may be overly conservative in many instances. Studies have demonstrated that environmental factors significantly affect thermal expansion and contraction of a line, and thus its sag. Windy days cool lines through thermal convection, while cold days cool lines through thermal conduction. Sunny days heat lines through solar radiation. Static line ratings are developed with the conservative assumption that the ambient environmental conditions are exceptionally unfavorable to line cooling (i.e., relatively hot, sunny days, with little wind), despite the fact that this is most often not the case.